1. Field of the Invention
The present invention relates to a vehicle-installed microcomputer system having a function of processing sensor signals received from onboard sensors.
2. Description of Related Art
In recent vehicles, most of onboard devices are controlled by a microcomputer-based control system configured to convert analog sensor signals received from various sensors into digital signals, and compute the digital signals for the purpose of making necessary indication or alarm, or controlling external devices. Since the recent vehicles are provided with many sensors, such a microcomputer-based control system has many A/D converters for converting analog signals into digital signals.
The microcomputer-based control system including the A/D converters and the onboard sensors are supplied with electric power from a vehicle battery through a constant-voltage power supply circuit, as shown, for example, in Japanese Patent Application Laid-open No. 2004-280763, because the variation in the terminal voltage of the vehicle battery is relatively large.
The microcomputer-based control system is required to operate for a specific purpose such as security maintaining operation even after an ignition key is turned off to stop a vehicle engine. Meanwhile, there is a strong demand to reduce electric power consumption of the microcomputer-based control system in view of a limited capacity of the vehicle battery. Accordingly, it is common that the microcomputer is configured to enter a sleep mode where it operates on small electric power when the ignition key is turned off.
In the sleep mode, the microcomputer cannot perform most arithmetic computations other than simple operations (reception of a timer signal or an awake signal, for example), because its clock frequency is set at a low value while the microcomputer is in the sleep mode. It is also known that the microcomputer temporarily changes from the sleep mode to an awake mode for performing essential operations each time the microcomputer has received the awake signal, or each time a certain time has elapsed after the microcomputer has last entered the sleep mode.
FIG. 7 shows a configuration of a conventional vehicle-installed microcomputer system including A/D converters 105 to 108, a CPU 109, an I/O (input/output) interface circuit 110, a backflow preventing diode 120 connected to a vehicle battery, and a constant-voltage power supply circuit 111. The reference numerals 101 to 104 denote sensor circuits outputting analog sensor signal to the A/D converters 105 to 108.
At least a part of this vehicle-installed microcomputer system may be implemented as a single chip processor as shown in FIG. 8. The single chip processor shown in this figure includes the A/D converters 105, 106, CPU 109, I/O interface circuit 110, backflow preventing diode 120, constant-voltage power supply circuit 111, and a voltage detector detecting a terminal voltage of a vehicle battery as the sensor circuit 101.
The number of the onboard sensors connected to the vehicle-installed microcomputer system is increasing more and more in recent years. Under such circumstances, there arises a problem in that the power consumption of the vehicle-installed microcomputer systems is not reduced much even when it enters the sleep mode. This is caused by the fact that the power consumption of the constant-voltage power supply circuit of the recent vehicle-installed microcomputer systems, which is mainly due to resistance loss therein, is large, because the constant-voltage circuit of the recent vehicle-installed microcomputer systems has such a large power supply capacity as is needed to be able to supply electric power to many onboard sensors. The large power consumption of the vehicle-installed microcomputer system while the vehicle engine is stopped causes the vehicle battery to become exhausted too soon.
The power consumption of the constant-voltage power supply circuit also depends on its output accuracy. A high-accuracy constant-voltage power supply circuit generally used for supplying power to high-precision analog sensor circuits consumes a very large amount of electric power compared to a moderate-accuracy constant-voltage power supply circuit generally used for supplying power to digital circuits. Accordingly, the burden on the vehicle battery while the vehicle engine is stopped can be eased greatly, if a moderate-accuracy constant-voltage power supply circuit is used instead of the high-accuracy constant-voltage power supply circuit for the vehicle-installed microcomputer system.
However, it is desirable to use the high-accuracy constant-voltage power supply circuit in terms of avoiding the control accuracy of the vehicle-installed microcomputer system from being lowered.